tests/periph_spi: let build fail without SPI
make: introduce and use periph_spi feature
Merge pull request #1809 from LudwigOrtmann/Makefile.features-SPI

make: periph_spi feature
Initial import of nrf24l01p lowlevel functions
[SQUASM ME] removed tests/test_spi
[SQUASH ME] changed delay after function call
[SQUASH ME] removed netdev includes from drivers/makefile
[SQUASH ME] update some small things to changes caused by netdev in another PR
[SQUASH ME] added a small REAMDE to describe the usage of the test
[SQUASH ME] whitelist arduino-due for nrf24l01p lowlevel test
[SQUASH ME] style fixes
added msg_send function from rx-interrupt to thread
added documentation and changed some functions return types
[SQASH ME] removed spi transfers during ISR
[SQUASH ME] addressed comments by @LudwigOrtmann
[SQUASH ME] complete documentation and ran asytle again
[SQUASH ME] changed txmode/rxmode to send command